a term used by muslims to describe facebook. Fitna is an islamic terms that means trials and tribulations, meaning God (Allah) will test you. So the use of facebook will increase the chances that a muslim will gain sin by doing something unpermissable (Haram) on it...thus fitnabook.

Bad muslim: Dude i met a fine hijabi on facebook!
Decent muslim with wrong intention: man i would stay away from that fitnabook...itll waste your time.